1. Field
The present disclosure relates generally to electrical power systems, such as systems for generating and distributing electrical power on an aircraft. The present disclosure relates more specifically to identifying undesired conditions, such as subtransient current conditions and overvoltage transients, in electrical power systems and protecting electrical power systems from such undesired conditions.
2. Background
Aircraft may employ various electronic devices and systems to perform various functions on the aircraft. Power for the electronic devices and systems on an aircraft may be provided by an aircraft power system. The aircraft power system may include a number of generators along with various power distribution and conversion systems. For example, a number of generators may be driven by the aircraft engines. Feeder lines may carry generated power from the generators to the various power distribution and conversion systems.
Power conversion systems on aircraft may include, for example, rectifiers and transformers. Rectifiers may be used to provide direct current (DC) power from the alternating current (AC) power provided by the generators. Transformers may be used to provide various levels of AC and DC power. Power distribution systems on aircraft may include AC and DC buses and various feeder lines for carrying power between the buses and between the buses and various loads on the aircraft.
It is desirable to protect the components of an aircraft power system from undesired conditions that may occur in the power system. An example of such an undesired condition is a fault condition that results in a subtransient fault current.
A fault current is a current in a power system resulting from a fault. A fault current may rise relatively quickly after a fault occurs and then may decline. A relatively short surge of current resulting from a fault may be referred to as a transient fault current. Subtransient fault current may refer to a portion of a transient fault current. For example, subtransient fault current may refer to the portion of a transient fault current that occurs immediately after a fault. For example, without limitation, subtransient fault current may be a transient current resulting from a fault in a power system that occurs during approximately the first half-cycle or one or more cycles after the fault occurs. Subtransient fault currents typically represent the highest level of transient currents resulting from a fault. The magnitude of subtransient fault currents may be three to four times the magnitude of normal steady state fault currents.
A subtransient fault current in an aircraft power system may be caused when a hard fault occurs on the main generator bus of the power system. Energy is stored in the generator air gaps and magnetics. When a hard fault occurs on the main bus, all this energy is discharged into the fault.
The duration of a subtransient fault current is typically relatively short. For example, a subtransient fault current may only exist for a few milliseconds after a fault occurs. In an aircraft power system, after this relatively short period, a generator control unit on the aircraft may regulate the current on the generator bus back to allowable fault current limits.
It is desirable that protective and power control devices in an aircraft power system are sized to withstand undesired conditions which may occur in the aircraft power system, such as subtransient fault currents and other undesired conditions. As the power generating capabilities of aircraft power systems increase, it is desirable to increase the capability of the protective and power control devices used in aircraft power systems to resist higher fault current levels.
The ability to withstand higher fault current levels may be achieved by increasing the size of the protective and power control devices in an aircraft power system. However, using larger protective and power control devices in aircraft power systems may increase aircraft weight, which may increase aircraft operating costs in an undesired manner. Furthermore, in the past, suppliers have used contact materials with undesired characteristics to withstand subtransient fault currents. As an alternative, fusible links may be used to provide backup protection for protective and power control devices in aircraft power systems.
An overvoltage transient is another example of an undesired condition that may occur in an aircraft power system. An overvoltage transient is a relatively large and sudden spike in voltage in the power system. An overvoltage transient in a power system may be caused by various events.
An overvoltage transient may be the result of fault clearing in a power system. For example, a momentary voltage spike may occur at a location in a power system when a fault downstream from the location is cleared by a contactor downstream from the location. Such an overvoltage transient may occur even if the generator in the power system is de-excited almost immediately by a generator control unit.
As another example, an overvoltage transient in a power system may be caused by a lightning strike. The duration of an overvoltage transient caused by a lightning strike typically may be shorter than the duration of an overvoltage transient resulting from fault clearing in the power system. The energy of an overvoltage transient resulting from a lightning strike may be relatively limited due to the shorter duration of the transient.
It is desirable to protect the components of an aircraft power system from subtransient currents, overvoltage transients, and other undesired conditions.
Therefore, it would be desirable to have a method and apparatus that takes into account at least some of the issues discussed above, as well as other possible issues.